The trial judge, Mr Justice Keith, jailed the brothers, aged 10 and 11 at the time of the attack, for a minimum of five years last month. But the sentences were indeterminate, meaning they can only be released if the authorities believe they pose no threat to society.

"The judge was clearly correct to impose indeterminate sentences of detention and I agree with his analysis and with the minimum terms he set," she said.

The crime was the most notorious committed by British children since James Bulger's murder in 1993. The brothers targeted their victims apparently at random as they rode their bikes in Edlington on a Saturday morning.

The victims, aged nine and 11 at the time, were stripped, choked, stamped on, hit with bricks, made to eat nettles and forced to sexually abuse each other during the 90-minute ordeal in a secluded spot near Doncaster in April.

Phoenix Survivors, which campaigns for justice for victims of childhood sexual abuse, also criticised the five-year tariff.

Scotland said she accepted that the case was "truly shocking", but underlined that five years was the minimum term and the brothers would only be released when the risk they pose to the public is seen as "acceptable".

"In this case the judge approached the sentencing exercise with care and, after looking at all the factors involved, I do not consider the terms to be unduly lenient," she said. "I do want to emphasise an important point made by the judge, which is that five years is the very least these boys will serve."

Keith told the boys during sentencing that their the crimes as "truly exceptional" and carried out "for no reason other than that you got a real kick out of hurting and humiliating them".

A serious case review found that various agencies missed 31 opportunities to intervene with the boys' family. After sentencing, Doncaster council apologised to the victims and their families for its failings in the case.